Process of swelling cellulose fiber sheets



Patented Oct. 3, 1939 UNITED STATES PATENT OFFICE George A. Richter and Kenneth E. Glidden, Berlin, N. H., assignors to Brown Company, Berlin, N. H., a corporation of Maine No Drawing. Application January 17, 1938, Serial No. 185,339

12 Claims.

This invention relates to the swelling of cellulose fiber sheets with caustic soda solution and has for its objective the realization of maximum swelling of the sheets, as determined by increased thickness and/or by absorption of solution by the sheets.

The swelling of cellulose fiber sheets to a maximum or optimum degree by caustic soda solution is desirable when the sheets are to be used for 0 various purposes or are to undergo various aftertreatments, for maximum swelling generally bespeaks uniform or thorough action of the solution on the cellulose fibers or particles constituting the sheets. Thus, maximum swelling is desired in the case of the pulp-board or cellulose fiber sheets customarily used as raw material for the soda cellulose entering into the preparation of cellulose xanthate, cellulose ethers, and other cellulose derivatives. In such case, greater swelling of the sheets generally means more uniform impregnation and modification of the individual fibers constituting the sheets and smoother and more complete xanthation, etherification, or other chemical conversion of the 5 soda-cellulose shreds or crumbs into which the sheets are generally disintegrated preparatory to reaction with the converting chemical. Again, maximum swelling of a paper or cellulose fiber sheet is also desirable in those instances when the caustic soda solution is washed from the sheet and the sheet is dried preparatory to impregnation with various binder media, such as rubber latex, asphalt, resin compositions, lacquers, etc. In such instance, maximum swelling is generally accompanied by greater and more uniform extraction of non-alpha cellulose impurities from the fiber in the course of subsequent washing; and the dried sheet is more readily and completely impregnated with binder media by reason of its greater bulkiness and absorptivity. Even when a caustic-soda-solutionswollen paper or pulp sheet is to be compacted prior to washing and drying so as to yield comparatively dense or hard products, it is advantageous that swelling of the sheet by the solution take place to a maximum degree, for maximum swelling facilitates the subsequent realization of a dense and/or more or less translucent paper product or hard, board-like products.

We have discovered that swelling of a paper or pulp sheet by caustic soda solutions is phenomenally promoted when the sheet is thoroughly soaked or saturated with water prior to saturation with the causticv soda solutions, which are preferably of substantial or considerable strength, as will hereinafter appear. The primary saturation of the sheet with water and its secondary saturation with caustic soda solution may be conveniently accomplished by dipping 5 the sheet in successive baths of water and caustic soda solution for appropriate periods of time. Sufiicient caustic soda solution is used in the bath for the secondary dip and the water-soaked sheet can be submersed in the secondary bath sufficiently long so that, as removed from the secondary bath, the sheet is thoroughly saturated with caustic soda solution of the desired strength, say, a strength practically equal to that of 'the secondary bath prior to the dipping of the water-soaked sheet thereinto. In other words, the ratio of the weight of the solution serving in the secondary bath to the dry weight of the sheet and/or the water with which it is soaked is preferably sufiiciently great so that very little dilution-of the bath has occurred when the bath solution has completely displaced the aqueous medium present in the sheet. It is evidently the case that maximum swelling of the sheet is fostered because of the powerful osmotic forces at play as caustic soda from the bath solution diffuses into and throughout all the Watersoaked fibers or fiber walls. constituting the sheet,

There are widely varying conditions under which the two-step dipping process of the present invention may be accomplished. According- 1y, while we shall hereinafter describe particular examples of procedure and the results secured thereby, it is to be understood that these pro cedures and results are simply illustrative of our invention, and that our invention may be embodied in many other specific procedures. Preliminarily, it might be noted that the dips presently to be described were performed with wood pulp sheets similar to those used as raw material in the viscose-rayon industry. The air-dry pulp sheets were immersed'in water for one-half hour, drained for one minute, and then immersed in a bath of caustic soda solution for a two-hour period, with a 25 to 1 ratio of solution to bonedry pulp prevailing in the bath. The swollen sheets removed from the secondary bath were allowed to drain for one minute after'removal from such bath, were weighed, and their thickness determined with an instrument which did not compress the swollen sheets during the thickness measurement.

Using water at 20 C. in the primary or water bath and 18% caustic soda solution at 20 C. in

the secondary dipping bath, the following results were secured:

Percent Percent weight swelling increase The foregoing swelling results are phenomenally superior to those results secured, namely, e19" percent swelling and 750 percent weight increase, when the primary water dip was omitted and similar air-dry pulp sheets were dipped directly in a bath of 18% caustic soda solution at 20 C. I

under similar conditions as those of the secondary dip of the procedure hereof described.

Even greater swelling of the sheets was realized when the two-step dipping process hereof was performed with a secondary dip of the water-soaked sheets in cold caustic soda solution. Thus, the use after the primary water-dip of a secondary dipping bath consisting of 15% caustic soda solution at 10 C. yielded the following results:

Percent Percent weight swelling increase Excellent results were also secured by the twostep dipping process hereof, using a secondary dipping bath consisting of 10% caustic soda solution at 10 C. Indeed, such results (given immediately below) compare favorably with the results secured when the secondary dip takes place in 18% caustic soda solution at 18 0., especially as regards the amount of solution absorbed by the sheets.

Percent Percent weight swelling increase The phenomenon of increased swelling of the sheets is also realized when various water-miscible organic liquids, such as acetone, are substituted in part or in entirety for water in the primary dip bath. Thus, when acetone was used as the primary dip bath and 18% caustic soda solution at 20 C. was used as the secondary dip bath, the results were as follows:

Percent weight increase Percent swelling disintegration sheets that are dipped into a bath of caustic soda solution at subnormal temperature, e. g. at temperatures, below about 10 C., and that are removed in fully swollen or compressed condition from the bath.

ing of the sheets represents 100 times the increase in sheet thickness divided by the original sheet thickness; and the percent weight increase of the sheets represents 100 times the weight of solution absorb-ed per unit Weight of bone-dry pulp therein.

When the sheets represent raw material for xanthation, etherification or the like, their swelling by the two-step dipping process hereof may advantageously involve the use of caustic soda solution of about 18% or greater strength at about room temperature in the secondary dip bath. The solution-swollen sheets may then be pressed to remove excess solution and thereby to produce alkali-cellulose of the caustic soda content desired for subsequent xanthation, etherification, or the like. The pressed sheets may be shredded and the alkali-cellulose shreds or crumbs be aged, as usual, before undergoing conversion into cellulose xanthate, cellulose ethers, or the like.

In transforming waterleaf paper by the process hereof into dense parchment-like condition or boards into hard dense condition, the secondary dip may advantageously be effected in caustic soda solution of less than about 18% strength at temperatures downwards of about 18 C. and the solution-swollen paper or board pressed, washed, and finally dried. Thus, the secondary dip solution may be used at subnormal temperatures down to its freezing point and its strength is preferably lower (down to about 5% as a minimum), the lower the temperature of its use. The use of cold caustic soda solution in the secondary dip bath promotes greater gelatinization of the cellulose fibers and much greater weakening of the sheet than does caustic soda solution at room temperature (say, 18 0.), wherefore, in order to avoid disintegration or rupture of the sheet, it is desirable to support the sheet during its immersion in the cold caustic soda solution, especially as it is being run continuously through such solution. Thus, when a continuous paper or pulp sheet is being subjected to the two-step dipping process hereof, it may be run progressively through successive baths of water and cold caustic soda solution as the sheet is being supported, as by'an endless wire-cloth conveyor or the like, at least while in the bath of cold caustic soda solution and up to the point at which it is pressed and thus strengthened sufficiently to remain intact in the absence of a support through the rest of the process. The pressed sheet may be Washed, as by hot or cold wash Water, repressed, and finally dried.

The invention hereof is applicable to paper or pulp sheets made from one or more kinds of cellulose fiber, such as unbleached or bleached sulphite pulp, kraft pulp, soda pulp, cotton, etc. In securing the desired marked swelling effect on the sheets by the two-step dipping process hereof, it is necessary to provide a secondary dip solution of substantial strength, for instance, at least about 5% strength; and it is preferable to correlate the particular strength of solution with a temperature of use at which it induces maximum or optimum sheet-swelling. While caustic soda solutions of greater than 18% strength may be employed, we have found that maximum swelling is realized with solutions of approximately 15 to 18% strength. Accordingly, there appears to be no real advantage, generally speaking, in using solutions of greater than about 18% strength at room temperature. Each solu-- tion of less than about 18% strength had been found to have its particular temperature below about 18' C. at which optimum or maximum swelling occurs for its particular strength or causticity, so that when solutions of less than about-18% strength serve as the secondary dip baths, it is preferable to maintain them at their particular optimum or maximum sheet-swelling temperatures.

When the secondary dip of the sheet occurs in cold caustic soda solution, the primary dip may be effected in water substantially as cold as the secondary dip solution, thereby putting the sheet in condition to undergo quickest swelling action by the secondary dip solution. However, inasmuch as the secondary dip is preferably accomplished in a tremendous excess of solution, the sheet acquires the temperature of the secondary dip solution so quickly even when the water-soaked sheet is at ordinary temperatures that the temperature of the primary dip water is a relatively unimportant matter. In this latter connection, it might be observed that the ratio of secondary dip solution to the dry weight of the sheet and/or its water content may vary widely, depending upon the particular strength of the secondary dip solution. Thus, when working with caustic soda solutions of strength, it is desirable that the ratio of such solution to the dry sheet be much larger than when working with a secondary dip solution, of, say, about 18% strength. Thus, the ratio of the secondary dip solution to the dry weight of the sheet and/or its water content may be such that comparatively little dilution of such solution occurs. In some instances, the caustic soda concentration of the solutidn in the secondary dip bath may be somewhat greater than that of the solution infused into the sheet as a result of its soaking in waterwet condition in the secondary dip bath. In other words, the caustic soda solution surrounding the solution-soaked and swollen sheet as it is removed from the secondary dip bath may be of greater strength (e. g. of greater than 18% strength) than the solution (e. g. of about 5% to 18% strength) present in the sheet as it is removed from the bath or after it has been seasoned sufiiciently to permit its caustic soda content to diffuse substantially uniformly throughout its water content.

From the foregoing examples of procedure, it can be seen that sheets swollen by the process hereof are tremendously increased in weight by the caustic soda solution absorbed thereby from the secondary dip bath and that they contain not more than of cellulose. Such solutionswollen sheets of not more than 10% cellulose content can readily be produced by the process hereof with a caustic soda content or weight of greater than 10%; and such cellulose and caustic soda contents are usually desired in the swollen sheets when such sheets are to be pressed, shredded into alkali-cellulose crumbs, and the crumbs xanthated, or when such sheets are to be disintegrated in the presence of more or less diluting water and subjected to a one-step xanthating reaction involving the admixture of liquid carbon'bisulphide with the substantially individualized fiber suspension in caustic soda solution resulting from the disintegration of such sheets in the presence of more or less diluting water. The caustic soda content of the solutionswollen sheets may, as already indicated, be present in the sheets in the form of a caustic soda solution of about 5% to 18% strength, the particular strength of solution present in the sheets depending upon the caustic soda content or weight desired in the swollen sheets and being readily made consistent with a caustic soda content of not less than 10% and a cellulose content of not more than 10% in the sheets when such latter caustic soda and cellulose contents are desired in the sheets. Because such alkalies as caustic potash, lithium hydroxide, etc., may be used for the purposes hereof in lieu of part or all of the caustic soda, it is to be understood that the process hereof might be performed with solutions of one or more of such alkalies and that such alkalies as caustic potash and lithium hydroxide are to be considered as the equivalents of caustic soda.

The optimum sheet-swelling results are realized when the primary dip medium is plain water, but such medium may be a dilute caustic soda solution rather than plain water. Consequently, the characterization of the primary soaking or dipping medium in the appended claims as being water should not be taken in an absolute or literal sense but should be construed as including dilute caustic soda or other alkaline solutions, that is, solutions of much lower strength or alkalinity than that of the caustic soda solution of the secondary dipping bath. The important or determining factor in the results secured by our two-step dipping process appears to be the differential in strength or causticity between the primary dipping medium and the secondary caustic soda solution of much greater strength. When the secondary dipping solution is of, say, about 18% strength, the causticity or alkalinity of the primary water bath is permissibly greater than when the secondary dipping solution is of lower than 18% strength, say, 5% strength. Since the presence of caustic soda and water-miscible organic liquid, such as acetone, in the primary water bath detracts from the swelling effects attainable by the two-step dipping process hereof, we prefer to use plain water in the primary dipping bath. It is to be understood, nevertheless, that the term water used in the appended claims in describing the primary dipping bath hereof is meant to include the presence in such bath of more or less acetone or other water-miscible organic liquids, which are considered to be equivalents of water and which might be used to replace the water substantially in entirety.

The two-step treatment of a dried cellulose fiber sheet may, in accordance with the present invention, involve an initial treatment of such sheet with water or its equivalent under conditions that do not conduce to complete saturation of the sheet. For instance, the water may be applied to the sheet by a spray or applicator roll so as to wet the sheet substantially uniformly .with a limited amount of water, say, approximately its own weight of water, as contrasted with steeping the sheet in a water bath and thereby soaking it with, say, more than twice its own weight of water. In any event, it is desirable in the process hereof to wet the dried sheet substantially uniformly with at least ap- 75 proximately its own 'weight of water, but preliminary wetting or soaking of the sheet with more water, say, more than twice its own weight of water, yields even better swelling results when the water-wet sheet is dipped into the secondary bath of caustic soda solution.

It is possible pursuant to our invention to use more than two dipping baths under various conditions, for instance, a primary plain water bath, a secondary bath of 5% to caustic soda solution at room temperature, and a tertiary bath of about to 18% caustic soda solution at room temperature. Because the swelling results possible of attainment from more than two dipping steps are not superior to those realized from two dipping steps, We prefer for the sake of simplicity and economy to use two dipping steps in our process, as hereinbefore described.

We claim:

1. A process which comprises dipping a cellulose fiber sheet in dry condition in water until saturated thereby, redipping the water-saturated sheet in caustic soda solution of about 18% strength at about room temperature until the content of said sheet has been replacedjby said solution, pressing the sheet to predetermined caustic soda content, and converting the soda-cellulose of the resulting sheet into a cellulose derivative.

2. A process which comprises dipping a cellulose fiber sheet in dry condition inwater until saturated thereby, redipping the water-saturated sheet in caustic soda solution of about 18% strength at about room temperature until the content of said sheet has been replaced by said solution, pressing the sheet to predetermined caustic soda content, shredding the sheet into soda-cellulose crumbs, and xanthating the resulting crumbs.

3. A caustic-soda-solution-swollen, interfelted cellulose fiber sheet containing not more than about 10% by weight of cellulose and not less than about 10% by weight ,of caustic soda,'said sheet having been produced by substantially uniformly wetting an interfelted cellulose fiber sheet in dry condition with water in amount at least approximating its own weight and then soaking the resulting water-wet sheet with caustic soda solution of at least about 5% strength under such condition of solution-tosheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

4. A caustic-soda-solution-swollen, interfelted cellulose fiber sheet'containing not more than about 10% by weight of cellulose and not less than about 10% by weight of caustic soda in the form of caustic soda solution'of, about 5% to 18% strength, said sheet having been produced by substantially uniformly wetting an interfelted cellulose fiber sheet in dry condition with water in amount at least approximating its own weight and then soaking the resulting water-wet sheet with caustic soda solution of at least about 5% strength under such condition of solution-tosheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

5. A caustic-soda-solution-swollen, interfelted cellulose fiber sheet containing not more than about 10% by weight of cellulose and not less than about 10% by weight of caustic soda solution of a strength consistent with said cellulose and caustic soda contents, said sheet having been produced by substantially uniformly wetting an 'wrinterfelted cellulose fiber sheet in dry condition with water in -amount at least approximating its own weight and then soaking the resulting waterwet sheet with caustic soda solution of a least about 5% strength under such condition of solution-to-sheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

6. A process of promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, which comprises substantially uniformly wetting such sheet in dry condition with water in amount at least approximating its own weight and then soaking the resulting water-wet sheet with caustic soda solution of at least about 5% strength under such condition of solutionto-sheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

7. A process of promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, which comprises soaking the sheet in dry condition with far more than its own weight of water and resoaking the resulting water-soaked sheet with caustic soda solution of about 5% to 18% strength at temperatures ranging from freezing point of the solution to about room temperature and under such condition of solution-to-sheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

8. A process of promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, which comprises soaking the sheet in dry condition with far more than its own weight of water and resoaking the resulting water-soaked sheet with caustic soda solution of about 18% strength at room temperature and under such condition of solution-to-sheet ratio as to infuse such sheet throughout with caustic soda solution of at least. about 5% strength.

9. A process of promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, which comprises soaking the sheet in dry condition with far more than its own weight of water and resoaking the resulting water-soaked sheet with caustic soda solution of at least about 5% strength at temperatures downwards of room temperature to the freezing point of the solution and under such condition of solution-to-sheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

, 10. A process of promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, which comprises dipping the sheet in dry condition in water until saturated with far more than its own weight of water and then redipping the resulting water-saturated sheet in caustic soda solution of at least about 5% strength under such condition of solution-towater ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength.

11. In a process that involves promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, those steps which comprise dipping the sheet in dry condition in water'until saturated with far more than its own weight of water, redipping the resulting watersaturated sheet in caustic soda solution of at least about 5% strength under such condition of solution-to-water ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength, pressing the resulting solution-infused sheet, washing it, and drying the washed sheet.

12. In a process that involves promoting swelling action by caustic soda solution on an interfelted cellulose fiber sheet, those steps which comprise dipping the sheet in dry condition in water until saturated with far more than its own weight of water, redipping the resulting water- 10 saturated sheet in caustic soda solution of at least about 5% strength at temperatures downwards of room temperature to the freezing point of the solution and under such condition of soluticn-to-sheet ratio as to infuse such sheet throughout with caustic soda solution of at least about 5% strength, pressing the resulting solution-infused sheet, washing it, and drying the washed sheet.

GEORGE A. RICHTER.

KENNETH E. GLIDDEN. 

